Line signaling is a class of telecommunications signaling protocols

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

. Line signaling is responsible for off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

, ringing signal, answer, ground start

Ground start

In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone...

, on-hook

On-hook

In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

 unidirectional supervision messaging in each direction from calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 to called party

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 and vice versa. After an off-hook, line signaling initiates register signaling

Register signaling

In telecommunications, register signaling provides addressing information, such as the calling and/or called telephone number. R2 register signaling is an example.This is contrasted with line signaling....

 to accomplish the exchange of telephone number

Telephone number

A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

s of called party and in more modern line-signaling protocols, the calling party as well. While register signaling occurs, line signaling remains quiescent unless the calling party goes on-hook or an abnormal cessation of the call occurs, such as due to equipment malfunction or shutdown or due to network

Telecommunications network

A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...

 outage upstream in that call-attempt's series of spanned trunks.

Line signaling can be conveyed in a single DS0 channel of a trunk

Trunking

In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...

. In modern PCM telecommunications, line signaling is represented by the ABCD bits in DS0 #16 of an E1

E-carrier

In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

 or a selected DS0 within a T1

Digital Signal 1

Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

.

Line signaling can also be conveyed within the channels being supervised, as in the original T1 scheme which used one out of every eight bits for supervision, or with robbed bits of a superframe format, or as in-band signaling

In-band signaling

In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...

, e.g. as in L1 signaling where 2280 Hz tone pulses are used.